Automatic doors are used extensively in commercial, industrial and residential applications, where it is desired to provide a portal which can be covered and uncovered in a hands-free manner. In most instances, however, it is important that the opening be closed immediately after a person has passed through the portal, to prevent entrance of debris and further to retain environmental differences between the interior and exterior regions. Walk-in freezers, for example, are equipped with automatic doors to allow a person encumbered with parcels or pushing a load truck to easily enter, while at the same time minimizing the time during which the cool interior air may escape.
Doors which close automatically after a preset time present the possibility that an object or person may still be crossing the threshold when the doors begin to close. It is undesirable to strike a person passing through the portal, and it is also detrimental to the door mechanism to be driven against an obstacle toward a closed position. Hence, doors are provided with safety attachments which detect the presence of a person or obstacle in the door path, and reverse the direction of the door upon encountering abnormal resistance or being subjected to a force in a direction counter to the direction of closure. In many applications some such detector is required by law.
One type of safety edge utilizes a pneumatic signal tube which is enclosed within a flexible covering and which is supported within the tube by foam strips. Compression of the foam to such an extent that the tube is deflected causes a switch to be thrown which in turn reverses the motion of the door.
Another type of safety edge has a flexible edge strip which supports two conductive plates in space relation. Compression of the edge strip causes contact between the two plates, and hence reversal of door motion. In a variation on this approach, the edge strip itself may be made electrically conductive, and a conductive rib may extend within the strip to make contact with the strip when an obstacle is encountered.
Other doors use photo-electric sensors which detect blockage of the passage along the edge strip.
What is needed is a sensitive safety edge which is at the same time low cost and simple to install, and which when used with hi-part or overhead doors may also serve as a gasket or weather strip.